POS0401 OSTEOARTHRITIS SYNOVIAL MACROPHAGES HAVE A TOLERIZED PHENOTYPE AND A VERY WEAK CORTICOSTEROID RESPONSE
Authors: C. Wang, R. De Francesco, L. Lamers, S. Rinzema, S. Frölich, P. Van Lent, C. Logie, M. Van den Bosch
Journal: Annals of the Rheumatic Diseases (2023)

<h3>Background</h3> The majority of osteoarthritis (OA) patients show synovial inflammation. Intra-articular corticosteroid injections are often given to decrease this inflammation. Whereas these provide relatively short-term alleviation of clinical symptoms, they do not improve long-term disease prognosis. Monocytes/macrophages are key cells in OA synovial inflammation. Whereas past research mainly focused on pro-inflammatory M1-like and anti-inflammatory M2-like macrophages, a second logic related to long-term macrophage polarization is interesting to study; macrophage training versus tolerization that is the result of prior exposure of monocytes/macrophages to different inflammatory cues. This process is also referred to as innate immune memory. <h3>Objectives</h3> Here, we investigated what could underlie the absence of long-term amelioration of OA by corticosteroids. Hereto, we compared global RNA profiles of OA synovium macrophages with well-defined <i>in vitro</i> trained and tolerized monocyte-derived macrophage models, all in the presence or absence of corticosteroids. <h3>Methods</h3> Macrophages were obtained from OA synovium (OA-Mf) using tissue digestion followed by CD14+ MACS isolation or differentiated from blood-derived CD14+ monocytes using medium with human pooled serum. Tolerance and training was induced in monocyte-derived macrophages using 5 ng/mL LPS (LPS-Mf), mimicking macrophage paralysis as observed in sepsis, or 10 μg/ml β-glucan (BG-Mf), which mimics non-specific vaccination against an ulterior lethal pathogen exposure by generating macrophages that present robust M1 responses upon TLR receptor stimulation, during the first 24 hours of differentiation respectively. Samples were treated with 1 µM triamcinolone acetonide (TA) or DMSO for 4 hours. The genome-wide transcriptomic response to corticosteroids was determined using RNA sequencing. <h3>Results</h3> TA exposure of trained BG-Mf and tolerized LPS-Mf significantly altered the expression of 201 and 257 RNA transcripts, respectively. In contrast, in OA-Mf only 12 RNA transcripts were significantly regulated, indicating a globally paralyzed TA response. Interestingly, evaluation of the IL1, and C-C motif chemokine ligand (CCL) and CXCL clusters of chemotactic factors showed close resemblance between OA and tolerized but not trained macrophages in terms of basal expression, although only one gene in these clusters (<i>IL36RN</i>) was significantly regulated by TA in the OA-Mf. This indicates that OA-Mf have a state that is related to the memory of inflammation as in the LPS-Mf for key pathways involved in OA and further confirms that the OA-Mf have a weak TA response. However, this did not extend to all genes significantly induced by LPS (n= 895), since OA-Mf clustered apart from the LPS-Mf and BG-Mf, indicating a distinct global pro-inflammatory mRNA expression profile when compared with both tolerized and trained monocyte-derived macrophages. Finally, we observed that OA-Mf have a matrix-metalloproteinase (MMP) expression profile that is consistent with a macrophage phenotype, since they express <i>MMP9</i>, <i>MMP14</i>, and <i>MMP19</i>, but furthermore very strongly express <i>MMP1</i>, <i>MMP2</i>, <i>MMP3</i>, and <i>MMP10</i> that are not expressed by our monocyte-derived macrophage models, highlighting their unique phenotype. Again, corticosteroid treatment of OA-Mf could not repress MMP expression. <h3>Conclusion</h3> OA synovial macrophages have an inflammatory polarization state that closely resembles the tolerized macrophage state in terms of relevant secreted cytokines and chemokines transcripts. Furthermore, they show a very weak corticosteroid response, which might explain why corticosteroid treatment fails to ameliorate long-term pathology and symptoms in OA patients. <h3>References</h3> NIL. <h3>Acknowledgements:</h3> NIL. <h3>Disclosure of Interests</h3> None Declared.